Electrophoretic characterization of protein interactions suggesting limited feasibility of accelerated shelf-life testing of ultra-high temperature milk

Abstract

Accelerated shelf life testing is applied to a variety of products to estimate keeping quality in a short period of time. The industry has not been successful to apply this approach to UHT milk due to a number of chemical and physical changes of milk proteins that take place during processing and storage. These changes were investigated applying accelerated shelf life principles on UHT milk samples containing 2 different fat levels using native-and SDSpolyacrylamide gel electrophoresis. UHT skim (SM) and whole milk (WM) samples were stored at 20, 30, 40 and 50 °C for 28 days. Irrespective of fat content, UHT treatment had a similar effect on electrophoretic patterns of milk proteins. At the start of testing, proteins were crosslinked mainly through disulphide and non-covalent interactions. However, storage at and above 30 °C enhanced protein crosslinking more via covalent interactions. Extent of aggregation appeared to be influenced by fat content as WM contained a greater amount in comparison to that in SM implying aggregation via melted and/or oxidised fat. Based on reduction in loss in absolute quantity of individual proteins, covalent crosslinking in WM was facilitated manly through deamidated residues and products of lipid oxidation, while Maillard and dehydroalanine products were the main contributors involved in protein changes in SM.Protein crosslinking appears to follow a different pathway at higher temperatures (≥ 40 °C) to that at lower temperatures, which may make it very difficult to extrapolate these changes to predict protein crosslinking at lower temperatures.

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